This invention relates to a process for catalyzing or conducting a chemical reaction on a surface and the colloidal dispersion or solution used in the process. More particularly, the invention relates to a stable aqueous colloidal dispersion of positively charged polymer particles having diameters less than 3 micrometers, containing an active agent distributed throughout the polymer, and the use of the stable dispersion for depositing an adherent monolayer of the positively charged polymers on a surface followed by contacting the deposited monolayer with a suitable reactant. The catalytically active or reactive agent distributed throughout the polymers will be referred to hereinafter simply as an "active agent." The term polymer "particles" refers herein to water insoluble polymers containing an active agent distributed throughout as well as to water soluble polymers which become insoluble when an active agent is distributed throughout such soluble polymers. The polymers, whether initially soluble or insoluble in water, containing an active agent distributed throughout will be referred to hereinafter as a "catalyst" system. The catalyst system is particularly useful for catalyzing the complete electroless deposition of a conductive metal layer on the surface of a printed circuit board and the walls of the through-holes formed therein. The term "monolayer" as used herein refers to a surface coating or layer of the positively charged soluble polymer of one molecule thickness or to a layer of the positively charged polymer particles, containing an active agent, of one particle thickness. Monolayer includes both a partial or a complete coating of the surface by the adherent polymer.
Electroless deposition refers to the chemical deposition of a metal coating on a conductive, non-conductive, or semi-conductive substrate in the absence of an external electric source. Electroless deposition is used to apply metal coatings to: plastics, referred to as plating-on-plastics; metals, to provide corrosion and/or abrasion resistance; and is the preferred commercial method for depositing an adherent surface coating or predetermined pattern of a conductive metal, such as copper, on a dielectric substrate as in the manufacture of printed circuit boards. The key to accomplishing electroless deposition is to provide catalytic nucleating centers on the substrate at which centers or sites the reduction of metal ions to elemental metal and the deposition of the metal on the substrate occurs.
In the manufacture of printed circuit boards, a plastic panel, such as an epoxy/glass laminate, is used as the substrate. The substrate may have a metal foil, such as copper foil, laminated onto one or both of its surfaces, forming a metal cladding. When both surfaces are to be used to form a circuit thereon, connections are provided between the surfaces as by drilling or punching holes through the substrate at desired locations. In order to provide a continuous electrical path on and between the surfaces, the walls of the through-holes and the substrate surfaces must be made conductive as by electroless deposition. Following electroless deposition on the substrate surfaces and through-hole walls, the deposited metal layer is typically built up to an acceptable thickness for an electrical circuit, for example, by electroplating followed by photoresist imaging of the circuit.
Certain common processing steps are utilized in all conventional electroless deposition methods. The substrate surface must be carefully cleaned, etched, and rinsed. The substrate must also be conditioned for the deposition of catalytic nucleating sites. The conditioned substrate is then treated with a catalyst system for the deposition of catalytic nucleating sites on the substrate surface. The catalytic substrate is then accelerated to remove protective colloids which are typically used with the catalyst system to maintain the stability of the catalyst system. Electroless deposition of the desired conductive metal, such as copper or nickel, is then conducted by immersing the catalytic substrate in a solution of a salt of the electroless metal and a reducing agent. The electroless deposition then proceeds autocatalytically.
Since it is necessary to deposit a complete electroless metal layer on the substrate surface and through-hole walls, attempts have been made to provide a sufficient number of active catalytic nucleating sites thereon using stable conditioning systems and catalyst systems, while at the same time attempting to maximize the thickness of the electroless metal layer, reducing the process steps and time involved, minimizing the loss and deactivation of the catalytic metal, and obtaining reproducible results, and the like. While many improvements have been made over the past twenty years in these areas, because of increased quality requirements, such as for complete through-hole plating, the advent of multilayer printed circuits and improved techniques for examining the coverage of the electroless deposition on substrate surfaces and through-hole walls, it has been found that the conventional conditioning and catalyst systems are deficient for today's increasingly stringent requirements. One example of such a deficiency is the determination that incomplete throughhole wall coverage is obtained with conventional electroless deposition methods, and furthermore that incomplete through-hole wall deposition promotes outgassing, or the permeation of volatile materials from behind the deposited metal into the through-holes, resulting in blow-holes in the electroless metal layer during soldering of component leads to the circuit.
The present invention is directed to an improved electroless deposition process and catalyst system which offers significant improvements over conventional catalyst systems, especially in providing complete coverage of the substrate surface and through-hole walls with an adherent, thicker electroless metal layer while reducing the process steps and catalyst losses, among other advantages.